$140 million in sales expected this year

Twitter will earn $139.5 million in ad revenue this year, more than doubling the $45 million the site booked in 2010, according to a new forecast from eMarketer.  By 2013, eMarketer estimates Twitter’s ad revenue will reach nearly $400 million.

These numbers actually suggest a decrease in expected revenue this year. The lower projection is a result of a slower-than-expected rollout of several advertising initiatives, including a platform enabling advertisers to buy ads on a self-serve basis and sales offices outside the US. In January, eMarketer forecast that Twitter would have $150 million in ad revenue this year.

This year, 96% of ad revenue will come from the US. By 2013, US sales will account for 88% of Twitter’s ad sales and will rise to $351.6 million.